Adaptive power oscillation suppression controller for network converter

Abstract

The present disclosure provides a method and a control unit for controlling a power unit (14) connected to a power grid (16) at a connection point (20). The method includes obtaining a power oscillation suppression (POD) signal (POD) from a POD controller based on power oscillations in the grid. The method further comprises determining a gain value (G) based on a difference between a first voltage measurement value (U1) measured at the connection point and a first voltage reference value (Uref1) or an estimated short circuit ratio SCR of the power grid. The method further includes adding the POD signal to an input signal (In2) of a power unit regulator (108) to obtain an intermediate reference signal (refi) from the power unit regulator. The input signal is based on a reference value of the grid and a corresponding measured value of the grid. The method further comprises determining a combined reference signal (refc) based on adding a product (G, POD) of the POD signal and the gain value to the intermediate reference signal, and operating the power unit using the determined combined reference signal to suppress oscillations in the grid.

Description

Technical Field

[0001] The present disclosure relates generally to the field of power systems. In particular, the present disclosure relates to controlling power units. Background Art

[0002] Low-frequency electromechanical inter-area (power) oscillations are common in power systems and grids. These power oscillations raise concerns about the reliable operation of the system.

[0003] Different types of power units coupled to the grid, such as synchronous machines, power converters, or wind farms / wind power plants, can be used to dampen such inter-regional oscillations. Specifically, reactive power injection can be used to modulate the voltage at the connection point between the grid and the power unit. This function is usually implemented by an auxiliary control function of the power unit, usually called a power oscillation damping (POD) controller. Typically, the POD control signal is added to the voltage reference value of the AC voltage regulator of the power unit. Therefore, the POD signal is used to manipulate the combined voltage reference of the power unit to achieve damping.

[0004] In recent years, there has been an increasing demand for power converters that exhibit voltage source behavior (also called grid forming behavior). However, the requirements for grid-forming power converters, such as stability and a wide range of short-circuit ratios (SCR) to support the grid, may not be compatible with conventional POD controllers.

[0005] For example, in order to achieve stable performance of a wide range of SCRs, the outer voltage control loop of a grid-type converter is very slow. On the other hand, POD control may require a faster response time to counteract power oscillations in the grid.

[0006] Additionally, in certain applications, the goal of POD control is to modulate the PCC voltage to be in phase with the observed inter-area oscillations. However, conventional POD controllers typically include filters to generate the POD output. These filters often result in a phase difference between the input and output of the POD controller. The power unit control loop can provide further phase differences between the POD controller contribution and the actual output voltage modulation measured at the connection point. These phase delays may collectively complicate the goal of zero phase difference between the actual inter-area oscillations and the output voltage modulation. If the phase difference is too large, POD control may not be able to successfully suppress the inter-area oscillations and, in some cases, may result in undesirable higher frequency oscillations. Summary of the invention

[0007] It is therefore an object of the present invention to overcome at least some of the above disadvantages and to provide an improved method and controller for controlling a power unit.

[0008] This and other objects are achieved by the methods and devices defined in the attached independent claims. Further embodiments are defined by the dependent claims.

[0009] According to a first aspect of the present disclosure, a method for controlling a power unit connected to a power grid at a connection point is provided. The method includes obtaining a power oscillation suppression (POD) signal from a POD controller based on power oscillations in the power grid. The method also includes determining a gain value based on a difference between a first voltage measurement value and a first voltage reference value measured at the connection point or an estimated short circuit ratio (SCR) of the power grid. The method also includes adding the POD signal to an input signal of a power unit regulator to obtain an intermediate reference signal from the power unit regulator. The input signal is based on a reference value of the power grid and a corresponding measurement value of the power grid. The method also includes determining a combined reference signal based on adding the product of the POD signal and the gain value to the intermediate reference signal, and operating the power unit using the determined combined reference signal to suppress power oscillations in the power grid.

[0010] According to a second aspect of the present disclosure, a control unit for a power unit connected to a power grid at a connection point is provided. The control unit includes a power unit regulator. The control unit is configured to obtain a POD signal from a POD controller based on power oscillations in the power grid. The control unit is also configured to determine a gain value based on a difference between a first voltage measurement value and a first voltage reference value measured at the connection point or an estimated SCR of the power grid. The control unit is also configured to add the POD signal to an input signal of the power unit regulator to obtain an intermediate reference signal from the power unit regulator. The input signal is based on a reference value of the power grid and a corresponding measurement value of the power grid. The control unit is also configured to determine a combined reference signal based on adding the product of the POD signal and the gain value to the intermediate reference signal, and to operate the power unit using the determined combined reference signal to suppress power oscillations in the power grid.

[0011] The connection point may be, for example, a point of common coupling (PCC) or a point of coupling (POC).Therefore, the first voltage measurement value measured at the connection point may be equal to the PCC voltage.

[0012] The POD signal may be, for example, a voltage or reactive power signal. The POD signal may be obtained from a POD controller. Such a POD controller may be external to the control unit of the power unit. Alternatively, the control unit of the power unit may include a POD controller.

[0013] The POD signal is added to the input of the power cell controller. The power cell controller may be configured to provide an output, namely an intermediate reference signal, which is determined to minimize an error represented by the input signal. For example, the error may be the difference between a desired (reference) value and a measured value of the grid or the power cell. Adding the POD signal to the input of the power cell controller may provide an intermediate reference signal to minimize the power oscillations represented by the POD signal.

[0014] However, according to the present control method and the present control unit, the reference signal for operating the power unit, ie the combined reference signal, is further determined based on the product of the POD signal and the gain. For example, the POD signal may be fed back to the combined reference signal.

[0015] Specifically, the combined reference signal is determined by adding the product of the POD signal and the gain value to the intermediate reference signal. Therefore, the POD signal scaled by the adaptive gain may directly affect the combined reference signal. This can further reduce the phase difference between the combined reference signal and the POD signal.

[0016] The gain may be based on a difference between a first voltage measurement value measured at the connection point and a first voltage reference value.Such an increase in the determination of the combined reference signal may minimize a phase shift between the POD signal and the combined reference signal.

[0017] The first reference voltage may be an expected voltage at the connection point. The difference between the first voltage measurement value and the first voltage reference value may be referred to as a first voltage error hereinafter, because it represents the difference between the expected value and the actual (measured) value of the first voltage. Therefore, the gain may be adaptively changed based on the first voltage error.

[0018] Alternatively, the gain may be based on an estimated short circuit ratio (SCR) of the grid. The estimated SCR may be received from a dedicated SCR estimator. Such an SCR estimator may be external to the control unit of the power unit. Alternatively, the control unit of the power unit may include the SCR estimator.

[0019] Adjusting the gain based on the SCR can ensure that appropriate suppression is provided over a wide range of short circuit ratios (SCR). For example, the gain may be higher for a grid with a higher SCR.

[0020] The control method of the present disclosure can provide that low-frequency electromechanical power oscillations in the power grid are appropriately suppressed. In embodiments where the power unit is a grid-type power converter, power oscillations (or "oscillations") can be suppressed while maintaining the voltage source behavior of the converter over a wide range of SCRs. The present control method can be compatible with grid-connected control without (or at least less) undesirable interactions with voltage regulation. In addition, using the proposed adaptive POD, oscillations can be suppressed by a small phase difference between the actual oscillation and the voltage modulation, even if the short-circuit ratio varies greatly.

[0021] According to some embodiments, the determination of the gain value may include feeding the difference between the first voltage measurement value and the first voltage reference value or the estimated SCR into the first controller.The method may also include determining the gain value based on an output from the first controller.

[0022] According to some embodiments, the control unit may include a first controller. The control unit may also be configured to feed the difference between the first voltage measurement value and the first voltage reference value or the estimated SCR into the first controller and determine the gain value based on an output from the first controller.

[0023] Feeding the signal (ie the first voltage error or the estimated SCR) into the controller and basing the gain on the output of the controller may provide further controllability and adjustability of the gain adaptability. The controller may be, for example, a PI controller.

[0024] According to some embodiments, the method may further include retrieving a gain value from a lookup table based on an output from the first controller.

[0025] According to some embodiments, the control unit may include a lookup table, and the control unit may be further configured to obtain the gain value from the lookup table based on the output from the first controller.

[0026] Alternatively, the lookup table may be implemented without the first controller. In such an embodiment, the gain value may be obtained from the lookup table based on the first voltage error or the estimated SCR.

[0027] According to some embodiments, the power cell regulator may include a voltage regulator.

[0028] The method may also include obtaining a first voltage reference and a first voltage measurement, and determining an input to the power cell regulator based on the first voltage reference and the first voltage measurement.

[0029] The control unit may be further configured to obtain a first reference voltage and a first voltage measurement value, and determine an input of the power cell regulator based on the first voltage reference value and the first voltage measurement value.

[0030] The voltage regulator may be an automatic voltage controller.The voltage regulator may be configured to minimize the first voltage error, ie to provide a reference signal suitable for controlling the power unit such that the first voltage measurement value is close to the first voltage reference value.

[0031] According to some embodiments, the power cell regulator may include a reactive power regulator.

[0032] The method may further include obtaining a first reactive power measurement value and a first reactive power reference value measured at the connection point. The method may further include determining an input to a power unit regulator based on the first reactive power reference value and the first reactive power measurement value.

[0033] The control unit may be further configured to obtain a first reactive power measurement value and a first reactive power reference value measured at the connection point, and determine an input of the power unit regulator based on the first reactive power reference value and the first reactive power measurement value.

[0034] For example, the reactive power regulator may be configured to minimize a first reactive power error based on a first reactive power reference value and a first reactive power measurement value.

[0035] According to some embodiments, the oscillation may be a power oscillation in the range of 0.2-2 Hz.

[0036] It should be noted that other embodiments can be envisaged using all possible combinations of the features recited in the above embodiments. Therefore, the present disclosure also relates to all possible combinations of the features described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Exemplary embodiments will now be described in more detail with reference to the following drawings:

[0038] Figure 1 is a schematic diagram of a grid-connected power electronic converter connected to a power grid according to some embodiments;

[0039] Figure 2 is a block diagram illustrating a control unit for operating a power unit according to some embodiments; and

[0040] Figure 3 is a block diagram illustrating a control unit for operating a power unit according to some embodiments.

[0041] As shown in the figures, the sizes of components and partitions may be exaggerated for illustrative purposes, and thus, these components and partitions are provided to illustrate the general structure of the embodiments. Like reference numerals refer to like elements throughout. DETAILED DESCRIPTION

[0042] Exemplary embodiments will now be described more fully below with reference to the accompanying drawings, in which the currently preferred embodiments are shown. However, the present invention can be embodied in many different forms and should not be construed as limited to the embodiments described herein; on the contrary, these embodiments are provided to fully convey the scope of the present invention to those skilled in the art thoroughly and completely.

[0043] Figure 1 A power unit 14 is schematically shown coupled to a grid 16 at a connection point 20 .

[0044] The power unit 14 may be a converter. The converter may be arranged to convert (DC or AC) power from a power source into (AC or DC) power suitable for the grid 16. The converter may be a grid-type power converter that is controlled to simulate a voltage source connected to a coupling point via an impedance.

[0045] The power unit 14 may alternatively be a synchronous machine, a wind farm, or a wind power plant.

[0046] Figure 2 A control unit 100 for controlling the power unit 14 to provide power oscillation suppression (POD) to the grid 16 is schematically shown. The control unit 100 may be configured to execute the method according to the second aspect of the present disclosure.

[0047] The control unit 100 comprises a power cell regulator 108 which is configured to receive an input signal In2 and to provide an (intermediate) reference signal ref i The POD signal POD is obtained, for example, from a POD controller based on power oscillations observed in the grid 16. The POD signal POD may be based on measurements made at the connection point 20, for example voltage measurements or reactive power measurements. The POD signal POD is added to the input signal In2 so that the power oscillations represented by the POD signal POD are used in calculating / determining the (intermediate) reference signal ref i is taken into account by the power unit regulator 108.

[0048] In the voltage control mode, the power unit regulator 108 may be a voltage regulator, for example, an AC voltage regulator. The input signal In2 may be a voltage signal. For example, the input signal In2 may be a first voltage error signal.

[0049] In the reactive power control mode, the power unit regulator 108 may be a reactive power regulator. The input signal In2 may be a reactive power signal. For example, the input signal In2 may be a first reactive power error signal.

[0050] According to the method and control unit 100 provided in the present disclosure, a combined reference signal ref cTo operate the power unit 14. Specifically, the gain value G is determined based on the input In1 (in block 102). The input In1 may be a first voltage reference signal U ref1 and the first voltage measurement signal U1 measured at the connection point 20. Alternatively, the input I1 may be an estimated short circuit ratio (SCR) of the electrical network 16.

[0051] The POD signal POD is multiplied by a gain G to provide a modified POD signal G,POD. i and the modified POD signal G,POD to determine the combined reference ref c (In box 104).

[0052] Figure 3 An example embodiment of a control unit 200 in a voltage control mode is schematically shown. According to some embodiments, Figure 3 The illustrated embodiment includes a number of optional features. Although the features described below are shown in the same figure, it should be understood that different embodiments may include Figure 3 Some or one of the features shown or not included Figure 3 Features shown in .

[0053] First, as described above, the gain G is determined in block 102. The control unit 200 may include a first controller 210, which may determine the gain G based on the first voltage measurement value U1 and the first voltage reference value U ref,1 The difference between the two (i.e., the first voltage error) is used to provide an output, such as Figure 3 As shown. Alternatively, the first controller 210 may alternatively or additionally determine the output based on the estimated SCR. For example, the first controller 210 may be a PI controller. The gain G may be determined based on the output of the first controller 210.

[0054] In addition, the control unit 200 may further include a lookup table 212. The lookup table may associate different input values ​​with appropriate gain values ​​G. Figure 3 In the embodiment, the lookup table 212 is implemented with the first controller 210. In such an embodiment, the lookup table 212 can associate different output values ​​from the first controller 210 with appropriate gain values ​​G. Alternatively, the lookup table can directly associate the first voltage error or estimated SCR with the appropriate increment value G.

[0055] Figure 3 The control unit 200 is shown in a voltage control mode. In such an embodiment, the power unit regulator 108 may include a voltage regulator, such as an AC voltage regulator. The input of the voltage regulator is obtained by ref,1 Subtract the first voltage measurement value U1 from the output and add the POD signal UPOD In the embodiment shown, the POD signal U POD Based on voltage measurements at the grid.

[0056] Alternatively, the control unit may be in a reactive power control mode. In such an embodiment, the first regulator 108 may include a reactive power regulator. The reactive power regulator may be based on a first reactive power measurement value (Q1, not shown), a first reactive power reference value (Q ref1 , not shown), and POD signal (Q POD , not shown) to provide an intermediate reference signal ref i The POD signal may be based on reactive power measurements measured at the grid.

[0057] As mentioned above, based on the intermediate reference signal ref i and the modified POD signal G,POD to determine the combined reference signal ref c .exist Figure 3 In the above example, the combined reference signal U is determined ref,c Including the modified POD signal U G,POD Added to the intermediate reference signal U ref,i .

[0058] exist Figure 3 The intermediate reference signal U ref,i and the combined reference signal U ref,c However, in the case where the power unit 14 is configured to receive a current reference value, the combined voltage reference signal U ref,c It can be multiplied by the virtual admittance Y to provide a combined current reference signal I ref,c For example, the combined current reference signal may be used in a grid-type power converter that is controlled to emulate a voltage source connected to a connection point via an impedance.

[0059] Those skilled in the art realize that the present invention is by no means limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

[0060] Although features and elements are described above in particular combinations, each feature or element may be used alone without the other features and elements, or in various combinations with or without the other features and elements.

[0061] Furthermore, variations to the disclosed embodiments may be understood and effected by those skilled in the art in practicing the claimed invention by studying the drawings, the present disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The fact that certain features are cited in mutually different dependent claims does not mean that a combination of these features cannot be used to advantage.

[0062] Detailed list of embodiments

[0063] 1. A method for controlling a power unit (14) connected to a power grid (16) at a connection point (20), the method comprising:

[0064] Obtaining a power oscillation suppression POD signal (POD) based on oscillations in the power grid;

[0065] The gain value (G) is determined based on:

[0066] A first voltage measurement value (U1) and a first voltage reference value (U ref1 ), or

[0067] Estimated short circuit ratio SCR of the grid;

[0068] The POD signal is added to the input signal (In2) of the power unit regulator (108) to obtain the intermediate reference signal (ref i );

[0069] The combined reference signal (ref c );as well as

[0070] The power unit is operated using the determined combined reference signal to dampen oscillations in the grid.

[0071] 2. The method according to claim 1, wherein determining the gain value comprises:

[0072] feeding the difference between the first voltage measurement and the first voltage reference or the estimated SCR into the first controller; and

[0073] A gain value is determined based on an output from the first controller.

[0074] 3. The method according to item 2 further includes: obtaining a gain value from a lookup table based on an output from the first controller.

[0075] 4. The method according to any of the preceding claims, wherein determining the combined reference signal comprises adding a product of the POD signal and a gain value to the intermediate reference signal.

[0076] 5. A method according to any preceding claim, wherein the power cell regulator comprises a voltage regulator, and the method further comprises:

[0077] obtaining a first reference voltage and a first voltage measurement value; and

[0078] An input to a power cell regulator is determined based on the first voltage reference value and the first voltage measurement value.

[0079] 6. A method according to any one of clauses 1 to 4, wherein the power unit regulator comprises a reactive power regulator, and the method further comprises:

[0080] obtaining a first reactive power measurement value and a first reactive power reference value measured at the coupling point; and

[0081] An input to a power cell regulator is determined based on a first reactive power reference value and a first reactive power measurement value.

[0082] 7. A method according to any preceding claim, wherein the oscillations are power oscillations in the range of 0.2-2 Hz.

[0083] 8. A control unit for a power cell (14) connected to a power grid (16) at a connection point (20), the control unit comprising a power cell regulator (108), the control unit being configured to:

[0084] Obtaining a power oscillation suppression POD signal (POD) based on oscillations in the power grid;

[0085] The gain value (G) is determined based on:

[0086] A first voltage measurement value (U1) and a first voltage reference value (U ref1 ), or

[0087] Estimated short circuit ratio SCR of the grid;

[0088] The POD signal is added to the input signal (In2) of the power unit regulator (108) to obtain the intermediate reference signal (ref i );

[0089] The combined reference signal (G,POD) is determined based on the product of the POD signal and the gain value (G,POD) and the intermediate reference signal. ref c );as well as

[0090] The power unit is operated using the determined combined reference signal to dampen oscillations in the grid.

[0091] 9. The control unit according to item 8, further comprising a first controller, wherein the control unit is further configured to:

[0092] feeding the difference between the first voltage measurement and the first voltage reference or the estimated SCR into the first controller; and

[0093] A gain value is determined based on an output from the first controller.

[0094] 10. The control unit according to item 9, further comprising: a lookup table, the control unit being further configured to obtain a gain value from the lookup table based on an output from the first controller.

[0095] 11. A control unit according to any one of items 9 or 10, wherein the determination of the combined reference signal comprises adding the product of the POD signal and the gain value to the intermediate reference signal.

[0096] 12. A control unit according to any one of clauses 8 to 11, wherein the power unit regulator comprises a voltage regulator, the control unit being further configured to:

[0097] obtaining a first reference voltage and a first voltage measurement value; and

[0098] An input to a power cell regulator is determined based on the first voltage reference value and the first voltage measurement value.

[0099] 13. A control unit according to any one of clauses 8 to 11, wherein the power unit regulator comprises a reactive power regulator, the control unit being further configured to:

[0100] obtaining a first reactive power measurement value and a first reactive power reference value measured at the coupling point; and

[0101] An input to a power cell regulator is determined based on a first reactive power reference value and a first reactive power measurement value.

[0102] 14. A control unit according to any of items 8-13, wherein the oscillations are power oscillations in the range of 0.2-2 Hz.

Claims

1. A method for controlling a power unit (14) connected to a power grid (16) at a connection point (20), the method comprising: obtaining a POD signal (POD) from a power oscillation suppression POD controller based on power oscillations in the power grid; The gain value (G) is determined based on: A first voltage measurement value (U1) and a first voltage reference value (U ref1 ), or An estimated short circuit ratio SCR of the power grid; The POD signal is added to the input signal (In2) of the power cell regulator (108) to obtain the intermediate reference signal (ref i ), wherein the input signal is based on a reference value of the power grid and a corresponding measured value of the power grid; The combined reference signal (ref c ); as well as The power unit is operated using the determined combined reference signal to suppress the power oscillations in the grid.

2. The method according to claim 1, wherein: Determination of the gain value includes: feeding the difference between the first voltage measurement and the first voltage reference or the estimated SCR into a first controller; and The gain value is determined based on an output from the first controller.

3. The method according to claim 2, further comprising: The gain value is retrieved from a lookup table based on an output from the first controller.

4. A method according to any one of the preceding claims, wherein: The power cell regulator comprises a voltage regulator, and the method further comprises: obtaining the first reference voltage and the first voltage measurement value; and An input to the power cell regulator is determined based on the first voltage reference value and the first voltage measurement value.

5. The method according to any one of claims 1 to 3, wherein: The power unit regulator comprises a reactive power regulator, and the method further comprises: obtaining a first reactive power measurement value and a first reactive power reference value measured at the coupling point; and An input to the power cell regulator is determined based on the first reactive power reference value and the first reactive power measurement value.

6. A method according to any one of the preceding claims, wherein: The power oscillation is a power oscillation within the range of 0.2-2 Hz.

7. A control unit for a power cell (14) connected to a power grid (16) at a connection point (20), the control unit comprising a power cell regulator (108), the control unit being configured to: obtaining a POD signal (POD) from a power oscillation suppression POD controller based on power oscillations in the power grid; The gain value (G) is determined based on: A first voltage measurement value (U1) and a first voltage reference value (U ref1 ), or An estimated short circuit ratio SCR of the power grid; The POD signal is added to the input signal (In2) of the power cell regulator (108) to obtain an intermediate reference signal (ref i ),in, The input signal is based on a reference value of the power grid and a corresponding measured value of the power grid; The combined reference signal (ref c ); as well as The power unit is operated using the determined combined reference signal to suppress the power oscillations in the grid.

8. The control unit according to claim 7, further comprising a first controller, wherein: The control unit is further configured to: feeding the difference between the first voltage measurement and the first voltage reference or the estimated SCR into the first controller; as well as The gain value is determined based on an output from the first controller. 9 . The control unit of claim 8 , further comprising a lookup table, the control unit being further configured to obtain the gain value from the lookup table based on an output from the first controller.

10. The control unit according to any one of claims 7 to 9, wherein: The power unit regulator includes a voltage regulator, and the control unit is further configured to: obtaining the first reference voltage and the first voltage measurement value; and An input to the power cell regulator is determined based on the first voltage reference value and the first voltage measurement value.

11. A control unit according to any one of claims 7 to 9, wherein: The power unit regulator includes a reactive power regulator, and the control unit is further configured to: obtaining a first reactive power measurement value and a first reactive power reference value measured at the connection point; and An input to the power cell regulator is determined based on the first reactive power reference value and the first reactive power measurement value.

12. A control unit according to any one of claims 7 to 11, wherein: The control unit is configured to obtain the POD signal (POD) from the POD controller based on power oscillations in the grid within a range of 0.2-2 Hz.

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